Optimizing 4D abdominal MRI: image denoising using an iterative back-projection approach.

4D-MRI is a promising tool for organ exploration, target delineation and treatment planning. Intra-scan motion artifacts may be greatly reduced by increasing the imaging frame rate. However, poor signal-to-noise ratios (SNR) are observed when increasing spatial and/or frame number per physiological cycle, in particular in the abdomen. In the current work, the proposed 4D-MRI method favored spatial resolution, frame number, isotropic voxels and large field-of-view (FOV) during MR-acquisition. The consequential SNR penalty in the reconstructed data is addressed retrospectively using an iterative back-projection (IBP) algorithm. Practically, after computing individual spatial 3D deformations present in the images using a deformable image registration (DIR) algorithm, each 3D image is individually enhanced by fusing several successive frames in its local temporal neighborood, these latter being likely to cover common independent informations. A tuning parameter allows one to freely readjust the balance between temporal resolution and precision of the 4D-MRI. The benefit of the method was quantitatively evaluated on the thorax of 6 mice under free breathing using a clinically acceptable duration. Improved 4D cardiac imaging was also shown in the heart of 1 mice. Obtained results are compared to theoretical expectations and discussed. The proposed implementation is easily parallelizable and optimized 4D-MRI could thereby be obtained with a clinically acceptable duration.

In vivo MRI discrimination between live and lysed iron-labelled cells using balanced steady state free precession.

OBJECTIVES: The goal of this study was to evaluate the ability of balanced steady state free precession (b-SSFP) magnetic resonance imaging sequence to distinguish between live and lysed iron-labelled cells. METHODS: Human breast cancer cells were labelled with iron oxide nanoparticles. Cells were lysed using sonication. Imaging was performed at 3 T. The timing parameters for b-SSFP and the number of iron-labelled cells in samples were varied to optimise the b-SSFP signal difference between live and lysed iron-labelled cell samples. For in vivo experiments, cells were mixed with Matrigel and implanted into nude mice. Three mice implanted with live labelled cancer cells were irradiated to validate this method. RESULTS: Lysed iron-labelled cells have a significantly higher signal compared with live, intact iron-labelled cells in bSSFP images. The contrast between live and dead cells can be maximised by careful optimisation of timing parameters. A change in the b-SSFP signal was measured 6 days after irradiation, reflecting cell death in vivo. Histology confirmed the presence of dead cells in the implant. CONCLUSIONS: Our results show that the b-SSFP sequence can be optimised to allow for the discrimination of live iron-labelled cells and lysed iron-labelled cells in vitro and in vivo.